1. Field of the Invention
This Invention relates to a real image mode zoom finder suitable for a lens shutter camera or a still camera in which a finder lens system is constructed to be independent of a photographic lens system.
2. Description of Related Art
In general, finder optical systems used in lens shutter cameras and constructed to be independent of photographing optical systems are roughly divided into two classes: virtual image mode finders and real image mode finders.
The virtual image mode finder has the disadvantage that the diameter of a front lens must be enlarged as the variable magnification ratio of the finder increases, which constitutes an obstacle to compactness of the finder. Furthermore, this type of finder optical system has the problem that the view of a field frame is unclear.
The real image mode finder, in contrast with this, is such that the diameter of the front lens can be diminished because its entrance pupil can be located on the front side of the finder. Since an image formed by an objective system can be observed through an ocular system, the defects of the virtual image mode finder mentioned above are practically obviated, and thus a finder with a good view of the field frame can be obtained. Moreover, in the real image mode finder, it is possible to realize a smaller-sized finder by modifying the configurations of prisms constituting an image erecting system. It is for these reasons that such real image mode finders are used at present in many of lens shutter cameras having variable magnification functions.
In recent years, further compactness of the camera and an improvement on the variable magnification ratio of the finder have been required. For the real image mode finder, a two- or three-lens unit type zoom system has been proposed as a finder with a variable magnification ratio of about 2. In order to further increase the variable magnification ratio, it is necessary to improve the variable magnification functions of individual lens units constituting the finder. Consequently, the refracting powers of the individual lens units of the finder are strengthened and the fluctuation of aberration caused when the magnification of the finder is changed becomes heavy. Furthermore, in order to change the magnification, it is necessary to hold sufficient space for movement of the individual lens units. This increases the overall length of the finder and the thickness of the camera.
If the overall length of the finder is reduced with little change of the variable magnification ratio, the refracting powers of the individual lens units must be made stronger because the space required for lens movement where the magnification is changed becomes narrow, and considerable aberration will be produced by the lens units for changing the magnification.
In this way, when the variable magnification ratio of the real image mode finder constructed with two or three lens units is increased, considerable aberration is particularly produced by the lens units for changing the magnification, and hence the fluctuation of aberration caused when the magnification is changed becomes prominent. In the two- or three-lens unit type zoom system, the number of lens units is so small that correction for aberration will be difficult if at least one lens unit which yields a considerable amount of aberration is included in the finder.
These problems, however, are solved when the zoom system of the finder is constructed with four lens units. According to the zoom system of the four lens units, functions relative to variable magnification and correction for aberration can be successfully imparted to the individual lens units. When the first lens unit closest to an object, of the four lens units, has a negative refracting power, the back focal distance of the objective system can be increased. In particular, this arrangement is very advantageous for compactness of the finder when a part of image inverting members is placed in the objective system. Furthermore, when the fourth lens unit has a negative refracting power, this arrangement is particularly advantageous for the case where the overall length of the objective system is reduced on the side of the secondary focal length.
For the zoom system of the finder constructed with the four lens units, a finder with a variable magnification ratio of at least 2, having an objective system which includes, in order from the object side, lenses with negative, positive, positive, and negative powers, is disclosed, for example, in each of Japanese Patent Preliminary Publication Nos. Hei 5-164964, Hei 7-128707, and Hei 8-240769.
In Hei 5-164964, a finder shown in the first embodiment is favorably corrected for aberration in such a way that the second lens unit of the objective system which has a leading, variable magnification function is constructed with a plurality of lenses. However, lenses constituting the finder are large in number, which causes oversizing of the finder. A finder described in the second embodiment of this publication is designed so that each of lens units is constructed with a single lens to thereby realize a small-sized finder. However, since in the third lens unit an aspherical surface with high refracting power is not placed on the image side thereof, but on the object side, there is room for improvement as to correction for aberration. In this finder, a combined longitudinal magnification of an optical system subsequent to the second lens unit of the objective system increases at the telephoto position of the finder. Thus, since the influence of the assembly error of the finder on a diopter becomes remarkable, there is the need to accurately assemble the finder, which causes an increase in production cost.
In Hei 7-128707, a real image mode finder is disclosed in which each of the lens units of the objective system is constructed with a single lens and which has a wide angle of field and is small in size. However, since in this finder the entrance surface of the first lens unit is configured as a convex surface, the entrance pupil is located inside the finder, and the lens diameter of the first lens unit enlarges.
A finder disclosed in Hei 8-240769 is designed to realize a finder which has a variable magnification ratio as high as at least 3 and is small in size. However, the combined longitudinal magnification of an optical system subsequent to the second lens unit of the objective system increases at the telephoto position, and thus the Influence of the assembly error of the finder on the diopter becomes pronounced. Hence, an accurate assembly is required for the finder, resulting in a rise of production cost.